1. Field of the Invention
The present invention relates to a thermosensitive recording material with light transmission properties, capable of forming colored images therein by a coloring reaction between an electron-donating coloring compound and an electron-accepting compound, more particularly to a thermosensitive recording material which can be used as a film of a block copy, that is, an image formation film for gravure printing, offset printing, and screen process printing; a block copy film for screen textile printing; an image formation film for use with an overhead projector (OHP); and an image formation film for use in the system of computer aided design (CAD).
2. Discussion of Background
There is widely known a thermosensitive recording material capable of producing a colored image therein by the coloring reaction between an electron donating compound (hereinafter referred to as a coloring agent) and an electron accepting compound (hereinafter referred to as a color developer).
Generally, such a thermosensitive recording material is prepared by providing on a support such as a paper, a synthetic paper or a plastic film, a thermosensitive recording layer which comprises as the main component a thermosensitive coloring composition. A colored image can be formed in the thermosensitive recording material by the application of heat thereto using a thermal head, thermal pen or laser beam. This type of recording material is advantageous over other conventional recording materials because the recording can be speedily achieved by a simple process without complicated development and image fixing steps, using a relatively compact device. Noise development and environmental problems can be minimized, and the manufacturing cost of the recording material is low. Owing to the above-mentioned advantages of the thermosensitive recording material, it is utilized in a wide number of fields such as electronic computer terminals, facsimile machines, and recorders and automatic vending machines for labels and tickets.
The above-mentioned thermosensitive coloring composition for use in the thermosensitive recording material comprises a coloring agent, and a color developer capable of inducing color formation in the coloring agent upon application of heat thereto. To be more specific, colorless to light-colored leuco dyes with a lactone or lactam, or a spiropyran ring are employed as the coloring agents; and a variety of acidic materials such as an organic acid and a phenolic compound are conventionally used as the color developers. The recording material employing the combination of the above-mentioned coloring agent and color developer is capable of producing colored images with clear tone, with maintaining a high degree of whiteness of the background. In addition, the weather resistance of the obtained colored images is superior.
In recent years, the thermosensitive recording system has replaced the conventional recording systems. Further, in line with the increase of demand for the thermosensitive recording system, there is an increasing demand for the improvement of quality of a thermosensitive recording material for use with the above-mentioned thermosensitive recording system. For example, there is a demand for a transparent or translucent thermosensitive recording material capable of producing an image with high image contrast, which can be used as an image formation film for the OHP, a diazo intermediate paper, a film for design drawing, and a block copy film for gravure printing, offset printing, and screen process printing.
A transparent thermosensitive recording medium is proposed in Japanese Laid-Open Patent Application 1-99879. To prepare such a transparent thermosensitive recording medium, a coating liquid of an emulsion dispersion comprising a coloring agent in microcapsule form and a color developer which is dissolved in an organic solvent that is insoluble or slightly soluble in water is coated on a transparent support.
Another method for preparing a transparent thermosensitive recording medium is disclosed in Japanese Laid-Open Patent Application 5-104859. According to this application, the first thing is to chose a solvent in which an organic acid to be employed as the color developer is slightly soluble or insoluble, and a coloring agent and a binder resin to be employed are soluble. A dispersion prepared by finely dispersing the organic acid serving as the color developer in the above-mentioned solvent, and a solution prepared by dissolving the coloring agent and the binder resin in the above-mentioned solvent are mixed and stirred, so that a coating liquid for a thermosensitive recording layer is obtained. The coating liquid thus obtained is coated on a transparent support, thereby obtaining a thermosensitive recording layer. Then, a protective layer mainly comprising a resin is provided on the thermosensitive recording layer in order to fill up the minute surface roughness of the thermosensitive recording layer. By this method, a thermosensitive recording medium with high transparency can be obtained.
In addition, a thermosensitive recording medium which is applicable to a block copy film for printing is proposed as disclosed in Japanese Laid-Open Patent Application 8-118811.
However, the matching properties of those conventional transparent or translucent thermosensitive recording media to the thermal head are almost ignored because too much stress is laid on the transparency of the recording media. For instance, a thermofusible material for use in the thermosensitive recording medium is fused during the thermal printing operation, and such fused dust is attached to the thermal head. As a result, the image thus obtained becomes blurred, and the thermal head is worn away and short-circuit occurs between the electrodes of the thermal head. In addition, there occurs the so-called sticking phenomenon that the thermosensitive recording medium sticks to the thermal head by the application of heat load thereto, and consequently, abnormal images are produced. As mentioned above, the head-matching problems of the conventional transparent or translucent thermosensitive recording medium are more serious than those of the general-purpose thermosensitive recording paper, so that the improvements are highly desired. Particularly, when the thermosensitive recording medium is used as a block copy film for printing, the improvement of the head-matching properties of the thermosensitive recording medium is one of the most important topics because dimensional accuracy of images independently formed on block copy films with different colors is required when these block copy films with different colors are incorporated.
From the functional viewpoint, it is desirable that the problem of the head-matching properties be compensated by the protective layer provided on the thermosensitive recording layer. A material with high heat resistance, excellent thermal releasability, and sufficient binding properties is preferably employed for the protective layer for the purpose of improving the head-matching properties. Furthermore, the protective layer works to improve the transparency of the recording medium, and impart the chemical resistance, water resistance, light resistance and wear resistance to the recording medium. Selection of the materials for the protective layer is therefore very important when the above-mentioned functions of the protective layer are taken into consideration.
Furthermore, there remains the problem of fogging, that is, one of the problems concerning any thermosensitive recording materials, and the improvement of the fogging is eagerly desired.